The purpose of this research project is to investigate the effects of trace heavy metals (0.01-1 ppm) on the apparent activity of certain enzymes, particularly ATO-dependent enzymes and degydrogenases, both of which comprise as much as 20% of the known enzymes in the living system. Preliminary results indicated that ATP-heavy metal complex is a competitive inhibitor of ATP-Mg ions in many ATP-dependent enzymes. Low concentrations of heavy metal ions were found to cause enzyme inhibitions by covalent interactions with the essential cysteinyl SH group in the active sites of enzymes. Unexpectedly, stoichiometric amount of reduced glutathione (GSH) (1:1 with respect to metal ions) reverses the enzyme inhibition caused by heavy metal ions. It is proposed the GSH can be a natural chelator for numerous metal ions in vivo.